The present invention relates to methods and compositions for treating subterranean formations. More particularly, the present invention relates to treatment fluids comprising gelling agents and crosslinking agents, and methods of using these treatment fluids in high-temperature fracturing operations.
Treatment fluids may be used in a variety of subterranean treatments, including, but not limited to, stimulation treatments and sand control treatments. As used herein, the term “treatment,” or “treating,” refers to any subterranean operation that uses a fluid in conjunction with a desired function and/or for a desired purpose. The term “treatment,” or “treating,” does not imply any particular action by the fluid or any particular component thereof.
One common production stimulation operation that employs a treatment fluid is hydraulic fracturing. Hydraulic fracturing operations generally involve pumping a treatment fluid (e.g., a fracturing fluid) into a well bore that penetrates a subterranean formation at a sufficient hydraulic pressure to create or enhance one or more cracks, or “fractures,” in the subterranean formation. “Enhancing” one or more fractures in a subterranean formation, as that term is used herein, is defined to include the extension or enlargement of one or more natural or previously created fractures in the subterranean formation. The fracturing fluid may comprise particulates, often referred to as “proppant particulates,” that are deposited in the fractures. The proppant particulates function, inter alia, to prevent the fractures from fully closing upon the release of hydraulic pressure, forming conductive channels through which fluids may flow to the well bore. After at least one fracture is created and the proppant particulates are substantially in place, the fracturing fluid may be “broken” (i.e., the viscosity of the fluid is reduced), and the fracturing fluid may be recovered from the formation.
Treatment fluids are also utilized in sand control treatments, such as gravel packing. In gravel-packing treatments, a treatment fluid suspends particulates (commonly referred to as “gravel particulates”) to be deposited in a desired area in a well bore, e.g., near unconsolidated or weakly consolidated formation zones, to form a gravel pack to enhance sand control. One common type of gravel-packing operation involves placing a sand control screen in the well bore and packing the annulus between the screen and the well bore with the gravel particulates of a specific size designed to prevent the passage of formation sand. The gravel particulates act, inter alia, to prevent the formation particulates from occluding the screen or migrating with the produced hydrocarbons, and the screen acts, inter alia, to prevent the particulates from entering the production tubing. Once the gravel pack is substantially in place, the viscosity of the treatment fluid may be reduced to allow it to be recovered. In some situations, fracturing and gravel-packing treatments are combined into a single treatment (commonly referred to as “frac pack” operations). In such “frac pack” operations, the treatments are generally completed with a gravel pack screen assembly in place with the hydraulic fracturing treatment being pumped through the annular space between the casing and screen. In this situation, the hydraulic fracturing treatment ends in a screen-out condition, creating an annular gravel pack between the screen and casing. In other cases, the fracturing treatment may be performed prior to installing the screen and placing a gravel pack.
Maintaining sufficient viscosity in these treatment fluids can be important for a number of reasons. For example, maintaining sufficient viscosity is a primary concern for particulate transport and/or to create or enhance fracture width. Also, maintaining sufficient viscosity may be important to control and/or reduce fluid-loss into the formation. At the same time, it may also be desirable to maintain the viscosity of the treatment fluid in such a way that the viscosity also may be easily reduced at a particular time, inter alia, for subsequent recovery of the fluid from the formation.
To provide the desired viscosity, polymeric gelling agents commonly are added to the treatment fluids to form viscosified treatment fluids. The term “gelling agent” is defined herein to include any substance that is capable of increasing the viscosity of a fluid, for example, by forming a gel. Examples of commonly used polymeric gelling agents include, but are not limited to, guar gums and derivatives thereof, cellulose derivatives, biopolymers, and the like. To further increase the viscosity of a viscosified treatment fluid, often the gelling agent is crosslinked with the use of a crosslinking agent. Conventional crosslinking agents usually comprise a metal ion that interacts with at least two gelling agent molecules to form a crosslink between them, thereby forming a “crosslinked gelling agent.” In some instances, treatment fluids comprising crosslinked gelling agents also may exhibit elastic or viscoelastic properties, wherein the crosslinks between gelling agent molecules may be broken and reformed, allowing the viscosity of the fluid to vary with certain conditions such as temperature, pH, and the like.
In high temperature applications, however, some viscosified treatment fluids may degrade and lose viscosity, especially those that are aqueous-based and comprise biopolymer-based gelling agents. Accordingly, various viscosity-increasing synthetic polymers have been developed for use in aqueous treatment fluids to achieve high viscosity and subsequently broken. While such synthetic polymers have achieved some success, crosslinking these fluids may be problematic. For example, in some instances, the gelling agent may “over-crosslink” in the presence of high concentrations of crosslinking agent, yielding a treatment fluid that is over-viscosified, difficult to break, exhibits syneresis (i.e., separation of liquid in a gel), or has other undesirable rheological properties. In addition, in some instances, the gelling agent may crosslink too rapidly, often before introduction into the subterranean formation, resulting in high friction pressure and gel shear degradation inside the tubing used to introduce the treatment fluid into the subterranean formation.